1. Field of the Invention
The present invention relates to semiconductor devices, particularly to a power semiconductor device.
2. Description of the Background Art
In a power semiconductor device including a semiconductor element such as an IGBT (Insulated Gate Bipolar Transistor), an outer circumferential junction region such as a guard ring is formed to surround the IGBT for the purpose of rendering the main breakdown voltage characteristics stable. This formation of an outer circumferential junction region causes the depletion layer to spread to the outer circumferential junction region when voltage is applied across the collector and emitter to alleviate the electric field strength at the pn junction of the emitter region. Accordingly, the collector-emitter current (ICES) and collector-emitter voltage (VCES) become stable.
Moreover, a semi-insulating silicon nitride film is formed on the outer circumferential junction region for the purpose of stabilizing the potential distribution at the outer circumferential junction region. This formation of a semi-insulating silicon nitride film of high resistance on the outer circumferential junction region (guard ring) will cause the flow of a small current at the semi-insulating silicon nitride film when voltage is applied across the collector and emitter of the IGBT. Accordingly, the potential at the region between respective portions of the guard ring located along the outer circumferential region is fixed, rendering the potential distribution at the outer circumferential junction region stable. A semi-insulating silicon nitride film is also expressed as a sinSiN film. A power semiconductor device including the semi-insulating silicon nitride film is disclosed in, for example, “Power Semiconductor Device and Power IC Handbook”, edited by The Institute of Electrical Engineers of Japan, CORONA PUBLISHING CO. LTD., 1996 (Non-Patent Document 1).
In general, a chip on which a power semiconductor element such as an IGBT or diode is provided is incorporated into a power module to be used for a power semiconductor device. In a power module, the power semiconductor device is sealed with silicone gel and the like. Application of voltage across the collector and emitter causes the plus ions and minus ions of the impurity ions in the silicone gel to gather at the ground side and the high potential side, respectively, to develop polarization.
By this polarization, the distribution of the electric field strength at the outer circumferential junction region is altered, leading to the possibility of unstable main breakdown voltage characteristics such as the collector-emitter current (ICES) and collector-emitter voltage (VCES).
Particularly, a power semiconductor device having a rated voltage that exceeds 6 kV is readily affected by impurities ions since the electric field strength according to the voltage applied across the collector and emitter is high. It was sometimes difficult to render the main breakdown voltage characteristics stable for such a power semiconductor device as compared to a power semiconductor device that has a relatively low rated voltage.